1. Field of the Invention
The present invention relates to a method for fabricating a gallium nitride (GaN) single crystal substrate, and more particularly, to a method for fabricating a GaN single crystal substrate, in which occurrence of cracks during separation of the GaN single crystal substrate from a sapphire substrate can be suppressed.
2. Description of the Related Art
Galium nitride (GaN), which has a large direct energy band gap of 3.39 eV, is known as a useful compound semiconductor for use in the manufacture of light emitting diode (LED) which emits a short wavelength light. Due to high vapor pressure of nitrogen at the melting temperature of GaN, liquid phase crystal growth needs high temperature (above 1,500xc2x0 C.) and high nitrogen pressure (exceeding 20,000 atmospheres) conditions, which makes this a non-viable mass production process. The resultant single crystalline GaN substrate is formed as a plate having a dimension of about 100 mm2, so that it is unsuitable to be used in the manufacture of a variety of devices.
A vapor phase growth technique using an alternative substrate (hetero-epitaxy), such as metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE), has been attempted to form a GaN substrate. As the alternative substrate, a sapphire substrate has been most frequently used because it has a hexagonal structure, like GaN, and is stable at high temperatures. Also, the sapphire substrate is cheap. However, due to the differences in lattice parameter (about 16%) and thermal expansion coefficient (about 35%) from GaN, a strain occurs at the interface between the GaN layer and the sapphire substrate, which causes defects and cracks to occur in the crystalline structure. As a result, it is difficult to grow high-quality GaN layer, and lifetime of a device formed on the GaN layer grown on the sapphire substrate becomes short. If a stress applied to the sapphire substrate and the GaN layer is isotropic, and if the stress exerted upon the sapphire substrate by the GaN layer is smaller than the yield point, the substrates are bent rather than cracked, so the side of the sapphire substrate is concave. As the thickness of the GaN layer increases, the curvature radius from the bending becomes smaller. In addition, the surface of the resultant GaN layer is rough, so that there is a difficulty in processing the surface of the GaN layer. The sapphire substrate and the GaN layer are tilted 30xc2x0 each other at the interface, so that cutting a device and manufacture of a resonator from the substrates are also difficult. To solve these problems, there is a need for a reliable method for fabricating a free-standing GaN substrate.
When a GaN layer is separated from a sapphire substrate, a mechanical process with diamond powder or a chemical etching technique has been applied. The mechanical process has a problem of cracking of the GaN layer. When the GaN is grown before the polishing, the stress applied to the sapphire substrate on which the GaN layer is grown is less than or equal to that which causes the strain to surpass the yield strength, so that bending rather than cracking occurs. However, as the sapphire substrate becomes thin by polishing, the balance of the strengths is broken, thereby causing cracks to occur in the sapphire substrate, and in turn in the GaN layer. A problem in applying the chemical etching technique lies in that there is no etchant which has a high etch rate and high selectivity with respect to the sapphire substrate.
An alternative approach is to use a laser in separating a GaN layer from a sapphire substrate. The GaN layer is grown on the sapphire substrate by HVPE and separated from the sapphire substrate by radiating a laser beam. The principle of this approach is based on the laser beam transmission and absorption properties of the sapphire substrate and the GaN layer in wavelengths below 365 nm. The side of the sapphire substrate on which the GaN layer is not grown is irradiated by a laser beam, GaN decomposes into Ga and N2 at the interface between the GaN layer and the sapphire substrate, which allows separation of the GaN layer from the sapphire substrate. Use of UV laser in separating a GaN layer from a sapphire substrate is disclosed by M.K. Kelly (Jpn. J. Appl. Phys. Vol. 38, L217, 1999), and W.S. Wong (Appl. Phys. Letter, Vol. 72, pp. 599, 1998). However, these approaches also have many problems.
According to the disclosure by K. K. Kelly, as shown in FIG. 1A, a GaN layer 6 is grown on a sapphire substrate 8 by HVPE, and the GaN layer 6 is fixed to a heater 2 using a metal binder 4. A laser light 9 is radiated onto the sapphire substrate 8 to separate the GaN layer 6 from the sapphire substrate 8. The metal binder 4 is removed from the separated GaN layer 6, thereby resulting in a complete GaN substrate.
According to the disclosure by W. S. Wong, after forming a stack of a sapphire substrate 18, a GaN layer 16, an epoxy adhesive 14, and a silicon substrate 11, a UV laser 19 is radiated onto the sapphire substrate 18 to separate the GaN layer 16 from the sapphire substrate 18. The epoxy adhesive 14 is removed from the separated GaN layer 16, thereby resulting in a GaN substrate.
However, the two methods described above do not involve removing unnecessary polycrystalline GaN layer stuck to the edge or backside of the sapphire substrate before the separation process. Thus, it is difficult to attain a crack-free 2-inch GaN substrate. Also, a 4 mm2 (for the result by W. S. Wong) or 100 mm2 (for the result by M. K. Kelly). Due to such a small crack-free area, the GaN substrates formed by the methods cannot be applied to form a device. In addition, use of the metal binder and epoxy adhesive complicates the overall process.
It is an object of the present invention to provide a method for fabricating a gallium nitride (GaN) single crystal substrate, in which occurrence of cracks in a GaN layer during separation of the GaN layer from a sapphire substrate is suppressed.
To achieve the object of the present invention, there is provided a method for fabricating a gallium nitride (GaN) single crystal substrate, the method comprising: forming a GaN layer on the front side of a sapphire substrate; heating the sapphire substrate at a temperature of 600-1,000xc2x0 C.; and separating the GaN layer from the sapphire substrate by radiating a laser beam onto the back side of the sapphire substrate.
It is preferable that the laser has a wavelength of 380 nm or less, and a power of 0.1-0.35 J/cm2.
After forming the GaN layer on the front side of the sapphire substrate, a polycrystalline GaN layer appearing on the back side and edge of the sapphire substrate may be removed. It is preferable that the polycrystalline GaN layer is removed by grinding.
It is preferable that the method for fabricating a gallium nitride (GaN) single crystal substrate further comprises forming a silicon oxide layer on the back side of the sapphire substrate, before forming the GaN layer on the front side of the sapphire substrate, and removing the silicon oxide layer from the back side of the sapphire substrate. It is preferable that the silicon oxide layer has a thickness of 100-1,000 nm. The silicon oxide layer may be removed by wet etching using a hydrofluoric acid (HF).